Who is Participating?

I'll call in ValentinoV to this question, as he knows more about SSRS then I do.

btw Post your entire data source T-SQL into this question, and specificy whether there are any IF blocks or dynamic SQL in it. SSRS, like SSIS, requires a 'contract' between a source and a target that does not depend on any conditions. So, if there is any code that may alter the source for whatever reasons, that will need to be changed to a static set.